Photometer lamp circuits providing a standard voltage



Aug 9, W49.

R. w. G. HUNT PHOTOMETER LAMP CIRCUITS PROVIDING A STANDARD VOLTAGE Filed Feb. 12, 1947 J DENSITY WEDGZ UNKNOWN LIGHT FIGZ.

ROBERT W G. HUNT INVENTOR W ATTORNEYS Patented Aug. 9, 1949 ApplicatiorrFebruary 12, wins-em! nofiiasifiss 'I1i Grbat' Bi-Itain Novemter 15, 1946 1 team. (01:. 88:23) =1 -'-Ihe present inventionrelates "tothe obtaining theta-mp filament reachesa-predetermined color of a predetermined" "standard brightness: "of" an temperature.

electric lamp-and more'particblarly "tm an arrangement whereby an electric 'incandescent lamp ma 'r be made to provide a standard brightness-"without the aid or electric measuring instruments. The invention finds its primary use in "photometers with which: illumination of unknown brightness is compared with a standard brightness in' any-well known manner.

An object of the present invention is the provision of" a-circuit arrangemem makin itf'p'ossible to cause a predeterminednurrenttofiow through, or a predetermined voltage applied to, a lamp without employinga *speeialmeasuring instrument.

Another object of the invention is the-provision of-a method for bringing alamp to a-- standard brightness which does not require a voltmeter.

Other objects and advantages of the invention will appear from the following description when read in connection with the accompanying drawing in which:

Fig. 1 is a circuit diagram of a simple arrangement embodying the invention; and

Fig. 2 is a graph illustrating the principle underlying the operation of the invention.

In many photometers an object whose brightness is to be measured is compared visually with a standard brightness, such as an electric lamp, contained in the photometer by altering in any desired manner the apparent brightness of either the object or the lamp until the two appear equally bright, the magnitude of the alteration being a measure of the brightness of the object. Other photometers and other apparatus require a light source of standard brightness and the arrangement and method described below are directed to securing a standard brightness regardless of the particular use to be made thereof.

In order for a lamp to provide a truly standard brightness, it must always operate at the same voltage. In the absence of a constant voltage source or a voltmeter, it is difiicult to insure this same voltage and many devices have been developed for this purpose. Among such devices are those in which a known resistance is removed from a lamp circuit to boost the energization of the lamp a certain amount above its glow level, or resistances having difierent temperature coefiicients are alternately inserted in a lamp circuit to determine when the current reaches a predetermined value, and those having built-in optical pyrometers for ascertaining when In accordance with thepresent invention two similar lamps are so arranged with respect to a photometer-head that a brightness match is obtained only at aunique level. lhis maybe ac- 'cor'nplished as shown in" Fig. 1= by-connecting two lamps-C and-*D im'parallel'across -a'battery 40 and 'its controlling --reisst'ance l l and connecting irrserieswith the 'Iamp-C a fixed resistance l 2-to low-en-tlie voltage Vc on thelamp- 0 below-the Voltage Vd-on the "lamp D sc that the lamp C will be--less brightthan lamp-D. No w;;ifthe resistance l I is adjusted to'givelamp -D its desired standard brightness and -thena photometeghead I3 is placed between the -two -lamp s C and D; in such-a Way-that the portions of the fields illumihated by the two lamps-are equallybright, as by inserting a-neutral=densityfilter in front ofthe lamp D or by placing the head '13 nearer to the lamp C than to the lamp D, any future adjustment of the resistance I I to match the two fields on the head l3 will automatically insure that the lamp D has the same brightness as was previously selected as standard.

The lamp C will, of course, in each case be brought to its former brightness and may comprise a standard different from that furnished by the lamp D,

The reason why the two lamps C and D will always operate at standard voltages whenever a match is obtained in the photometer head l3 will be clear from the graph of Fig. 2 in which the brightness Bd of that part of the photometer head field illuminated by the lamp D is plotted against the voltage Vd across the lamp D to obtain curve D, and the brightness Bc of that part of the photometer head field illuminated by the lamp C is also plotted against the voltage Vd to obtain the curve C. Since the two lamps C and D are similar the two curves would be similar in shape to curve D if both were plotted against the voltage Vc and Vd across the respective lamps C and D. But since the curve C is that of Be plotted against Vd (the voltage across the lamp C and the resistance [2) the shape of the curve C will be similar to that of curve D but will be stretched out along the voltage axis. Furthermore, since the head I3 is effectively or actually nearer to the lamp C than to the lamp D, the curve C will be magnified along the brightness axis. Thus, the curve C may be approximately described as being a magnified replica of the curved tail of the curve D, the magnification 3 along the brightness axis being greater than that along the voltage axis.

Under the circumstances described above it is clear that the curves C and D intersect at one point only and hence at one voltage Vs. At this standard voltage Vs, the two parts of the photometer head field are of equal brightness, i. e., Bc=Bd. Now any time the resistance H is adjusted to obtain a match in the photometer head l3 it is known that the voltag Vd across the lamp D has been adjusted to the standard value Vs and accordingly the brightnesses of the lamps C and D are standardized.

Inasmuch as the lamps C and D receive different voltages they will radiate at different colors. Such color difierence may be compensated by a filter l4 incorporated in the photometer head l3 or interposed between the head l3 and one of the lamps so that the two portions of the photometer field automatically match one another in color when the brightness match is obtained. Using ordinary flash lamp bulbs and a dry battery it has been found to be quite easy to standardize the brightness of the brighter bulb to within five per cent.

In practicing the invention the operator adjusts the resistance I I until a match is obtained on the photometer head I 3 at which time he knows that the two lamps C and D are at their predetermined brightnesses. The operator then compares the brightness of an object under investigation with one of the lamps. This comparison of the unknown light with one of the lamps may be made in any well known manner, one suitable arrangement being shown in Fig. 1 as comprising a comparison screen I5 upon the upper half of which the unknown light is permitted to fall. Light from the standard lamp, here shown as lamp D, is directed onto the lower half of the screen l5 by a, reflector [6 through a light- .4 modifying device which may be a density wedge H arranged in annular form for rotation with a shaft I8. The angular position of the wedge I! when a match is obtained on the screen 15 is of course a measure of the brightness of the unknown light; v

It will be understood that the drawing is diagrammatic and that only those parts are shown which are necessary to an understanding of the invention. Other arrangements may be employed in practicing the invention without departing from the scope of the appended claim.

I claim:

In a photometer, a photometer head, a photometric lamp for providing light of known brightness, optical means for directing light of unknown brightness and the light of known brightness into adjacent relationship for comparison purposes, means for adjusting the lamp to a predetermined brightness comprising means for varying the voltage applied to the lamp, 3, second lamp and a resistance connected in series across the terminals of the photometric lamp, whereby a predetermined brightness difference between the two lamps exists for a unique voltage, and photometric means for ascertaining when a desired brightness difierence exists between the two lamps.

ROBERT WILLIAM GAINER HUNT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,974,492 Helfenstein Sept. 25, 1934 2,217,236 Schafer Oct. 3, 1940 2,359,873 Poindexter Oct. 10, 1944 

